A 10 micro faran capacitor is charged by  a 30V dc supply & then connected across an uncharged capacitor of 50 micro faran. Calculate the(1) final potential difference across the combination & the (2)initial & final energies. How will you account for the difference in energies?

Capacitance on the charged capacitor C1= 10μF = 10 x 10-6 F

Supply Voltage = 30V

Capacitance of an uncharged capacitor C2 = 50μF = 50 x 10-6 F

When C2 is connected in the circuit the potential acquired by it is V2

According to conservation of charge, initial charge on capacitor C1 is equal to final charge on capacitors C1 and C2 .

Therefore V2 ( C1+ C2) = C1V1

Substituting the values

V2 = 5V

Initial energy of the system

E1 = ½ C1 V12

E1 = 45 x 10-4 J

Electrostatic energy of the combination of two capacitors is given by:

E2 =1/2 (C1 + C2 ) V22

E2 = 5 x 10-4 J

Hence the amount of electrostatic energy lost by the capacitor is

E= E1 – E2 = 40 x 10-4 J



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